Microlenses can be used in a variety of applications, such as in charge coupled devices (CCD), complementary metal oxide semiconductor (CMOS) image sensors, and liquid crystal displays (LCD). Microlenses can be formed as an array of microlenses on a supporting substrate, such as a semiconductor substrate. Typical microlenses include a very small lens used to focus light onto photosensitive elements. For instance, microlenses used in CCD arrays can be used to focus light onto photosensitive components of the CCD array. Similarly, microlenses can focus light into active regions in LCDs to generate images to be projected.
Microlenses can be formed on a semiconductor substrate using a plurality of semiconductor processing techniques. In one example, microlenses can be formed using photolithography techniques. Photolithography techniques alone, however, suffer several drawbacks. For instance, with the increasing miniaturization of electronic components, the amount of light of incident onto microlens arrays is becoming increasingly smaller, leading to reduced sensitivity. To increase the light collection efficiency of the microlens array, it is desirable to increase the lens size of each microlens while at the same time decreasing the distance between microlenses in the microlens array. Photolithography techniques alone are unable to achieve a sufficient reduction in the distance between microlenses in a microlens array, particularly in the diagonal direction.
For example, FIG. 1A depicts a top view of a microlens array 20 formed using known photolithography techniques. The microlens array 20 includes a plurality of microlenses 25 separated by an x-y distance D1 and a diagonal distance D2. As shown, in FIG. 1B, the x-y distance D1 between microlenses 25 can be reduced using a reflow process. The reflow process, however, only partially reduces the diagonal distance D2. The diagonal distance D2 is still large in terms of increasing the light collection efficiency of microlens arrays.
Etching techniques can be used to reduce the distance, including the diagonal distance, between microlenses in a microlens array. For example, relatively expensive etching gases, such as SF6 and C4F8, have been used to form microlenses having a larger area at a higher etching rate. In addition, etching gases such as CF4 in combination with other gases have also been used in capacitively coupled plasma etching processes to modify microlens shape in a microlens array. Capacitively coupled plasmas have higher plasma potential and do not provide for independent control of ion density and energy. As a result, the capacitively coupled plasmas tend to sputter the microlens surface leading to roughening of the microlens surface. The roughening of the microlens surface can cause scattering of light, reducing the light collection efficiency of the microlens.
Thus, a need exists for an improved method of forming microlenses that can increase the light collection efficiency of microlenses using more cost effective processing gases and etching tools.